Journal of Mammalian Evolution, Vol. 12, Nos. 3/4, December 2005 ( C  2005) DOI: 10.1007/s10914-005-5731-5 Organization of the Olfactory and Respiratory Skeleton in the Nose of the Gray Short-Tailed Opossum Monodelphis domestica Timothy B. Rowe, 1,2,3 Thomas P. Eiting, 1 Thomas E. Macrini, 1 and Richard A. Ketcham 1 The internal nasal skeleton in Monodelphis domestica, the gray short-tailed opossum, primarily supports olfactory and respiratory epithelia, the vomeronasal organ, and the nasal gland. This scaffold is built by the median mesethmoid, and the paired vomer and ethmoid bones. The meseth- moid ossifies within the nasal septum cartilage. The bilateral ethmoid segregates respiratory and olfactory regions, and its geometry offers insight into the functional, developmental, and genomic organization of the nose. It forms through partial coalescence of separate elements known as turbinals, which in Monodelphis comprise the maxilloturbinal, nasoturbinal, five endoturbinals, and two ectoturbinals. Geometry of the ethmoid increases respiratory mucosal surface area by a factor of six and olfactory mucosal surface by nearly an order of magnitude. Respiratory epithelium warms and humidifies inspired air, recovers moisture as air is exhaled, and may help mediate brain temperature. In contrast, the olfactory skeleton functions as a series of small funnels that support growth of new olfactory neurons throughout life. Olfactory mucosa lines the mouth of each funnel, forming blind olfactory recesses known as the ethmoid cells, and neuronal axons are funneled from the epithelium through tiny olfactory foramina in the cribriform plate, into close proximity with target glomeruli in the olfactory bulb of the brain where each axon makes its first synapse. The skeleton may thus mediate topological correspondence between odorant receptor areas in the nose with particular glomeruli in the olfactory bulb, enabling growth throughout life of new olfactory neurons and proper targeting by their axons. The geometric arrangement of odorant receptors suggests that a measure of volatility may be a component in the peripheral olfactory code, and that corresponding glomeruli may function in temporal signal processing. Supporting visualizations for this study are available online at www.DigiMorph.org. KEY WORDS: Vomeronasal organ, Mesethmoid, Ethmoid, Turbinals, Vomer, Computed tomography. INTRODUCTION Mammals live in an olfactory world to a greater degree than perhaps any other air-breathing creature. Olfaction mediates food detection, species isolation, social integration, myriad reproductive processes, alarm and defense, orientation and navigation, and a host of other 1 Jackson School of Geosciences, The University of Texas at Austin, Austin, Texas 78712, USA. 2 Texas Memorial Museum, The University of Texas at Austin, Austin, Texas 78712, USA. 3 To whom correspondence should be addressed at The University of Texas at Austin, Geol. Science Department, 1 University Station C1100, Austin, Texas 78712-0254, USA. E-mail: rowe@mail.utexas.edu 303 1064-7554/05/1200-0303/0 C  2005 Springer Science+Business Media, Inc.